问道sat数学~In the decimal represention of 1\k,where 0< 1\k < 1

问道sat数学~
In the decimal represention of 1\k,where 0< 1\k < 1 .the tenths digit is 1,the hundredths digit is 3,and at least one other digit is nonzero.what is the tenths digit in the decimal representation of k-1\k?
这题怎么算?

1/k = 0.13...其中后面一定有至少一个不为0的digit
(k-1)/k=1-1/k=0.86 or 0.87 不过无论如何,他的tenth digit是8.
所以答案是8